Variable expansion force stent

ABSTRACT

A stent having varying outward radial force along its length. In use, the stent can provide greater force in vessel regions requiring greater force and less force in regions requiring less. In particular, more force is provided in the narrowed, center of a stenosis, while not applying too much force to the adjoining healthy tissue area. Greater stent expansion is provided in wider vessel geometries and less stent expansion in narrower regions. Varying force is achieved varying the number of elements, the density of elements, and the thickness of the elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application from U.S. applicationSer. No. 09/193,504, filed Nov. 17, 1998 now U.S. Pat. No. 6,146,403which is a continuation application from U.S. Pat. No. 5,836,966corresponding to U.S. application Ser. No. 08/861,798, filed May 22,1997 the contents of both of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates generally to medical devices. More specifically,the invention relates to stents for holding vessels such as arteriesopen to flow.

BACKGROUND OF THE INVENTION

Stents are insertable medical devices used to maintain openings forfluid flow in areas that might otherwise close, hindering flow. Stentsare used to prevent restenosis after Percutaneous Transluminal CatheterAngioplasty (PTCA), presenting outward radial force against apotentially rebounding vessel wall after balloon widening. Stents arealso used to hold open inflamed vessel walls that would otherwise beswollen shut, precluding flow. Stents can also be used to hold opensurgically made holes for drainage.

Stents are often tubular devices for insertion into tubular vesselregions. Balloon expandable stents require mounting over a balloon,positioning, and inflation of the balloon to expand the stent radiallyoutward. Self-expanding stents expand into place when unconstrained,without requiring assistance from a balloon. A self-expanding stent isbiased so as to expand upon release from the delivery catheter.

A vessel having a stenosis may be modeled as an inwardly protrudingarcuate addition of hardened material to a cylindrical vessel wall,where the stenosed region presents a somewhat rigid body attached along,and to, the elastic wall. The stenosis presents resistance to anyexpansion of the vessel in the region bridged by the stenosis. Stenosesvary in composition, for example, in the degree of calcification, andtherefore vary in properties as well.

The arcuate geometry of many stenoses present a variation in resistancealong the vessel axis to stent outward radial force. Specifically,stenosed vessel resistance is often greatest toward the middle,lessening toward the ends, with a rapid decrease at the start of healthyvessel tissue.

A conventional self-expanding stent optimally has a length greater thanthe length of the stenosed region to be kept open. Current stentspresent a substantially uniform outward radial force along their length.Currently, stents do not vary outward radial force to match stenosisgeometries or resistances. A constant force stent, with sufficient forceto maintain an open channel within a stenosis, has greater force thannecessary in the healthy vessel portion lying past the stenosis ends.The stent ends may thus flare outward, protruding into, and possiblyirritating non-stenosed tissue.

Stenosis can occur in vessel regions having asymmetric geometry lying oneither side of the stenosis. One example of this is the ostium of acoronary artery, having a wide opening toward the aorta, converging intoa narrower coronary artery. A conventional stent placed in the ostiumwould provide substantially uniform outward force over a non-uniformvessel diameter. If this force is properly matched for the narrowervessel opening, it is likely less than optimal for the wider region.

What would be desirable, and has not heretofore been provided, is astent capable of providing sufficient force to keep a vessel open withina rebounding stenosis, while providing only necessary force againsthealthy, non-stenosed vessel regions. What also has not been provided isa stent providing necessary, but only necessary force along a stenosisin a vessel region having non-uniform vessel diameter on either side ofthe stenosis.

SUMMARY OF THE INVENTION

The present invention includes a self-expanding stent having a tubularshaped structure, where the outward radial force varies withlongitudinal position along the length of the stent. In one embodiment,the force is greater in the center and lesser at both ends. Such a stentis suitable for placement in a stenosed vessel region. In anotherembodiment, the force is less at one end, greater at the middle, andgreater still at the opposite end. Such a stent is suitable forplacement in a stenosed and narrowing vessel region, including placementnear a coronary ostium.

One stent has a structure formed of shape memory material. In oneembodiment, the stent is constructed of a Nickel-Titanium alloy.

The stent structure in a preferred embodiment includes a helix formed ofa wire having the helix turns spaced more closely together toward thecenter than at the ends. The helix is biased to expand in outer diameterand contract in length after having been stretched axially and released.In an alternate embodiment, the helix turns increase in spacing from oneend to the opposite end. In another embodiment, interwoven orintertwined wires form the tubular structure, with the number of wiresbeing greater per unit length toward the center than at the ends. Theinterwoven wires can be metallic wire. The wires can resemble spirals orhelices after having been wound to the tubular stent shape. In yetanother embodiment, the number of wires increase from one end to theopposite end.

One stent achieves a variation in radial force by including in the stentstructure elements which intersect at junctions having more material inregions requiring more radial force and less material in regionsrequiring less radial force. The amount of junction material can bevaried by varying the size of the junction area. In a preferredembodiment, the stent structure is formed by laser cutting a Nitinoltube, leaving a greater strut dimension in regions requiring greateroutward radial force.

In yet another embodiment, the stent structure includes a series of wiresprings having a “zig-zag” shape which each radially encircle a tubularsection. The springs are interconnected longitudinally. The requiredoutward radial force can be varied by varying the stent wall thicknessin this and other embodiments. In one embodiment, stent regionsrequiring greater radial force have thicker walls than regions requiringless force.

Stents made in accordance with the present invention can provide anoutward radial force more closely matching the local force requirements.In particular, the stents provide greater force only where required in astenosis center, without providing too much force in the region ofhealthy tissue. The stents provide an expanded geometry more closelytailored to the requirements of a narrowing vessel region, providinggreater expansion in wider regions and less expansion in narrowerregions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary longitudinal cross-sectional view of a stenosedvessel region;

FIG. 2 is a fragmentary cross-sectional view of a stenosed vessel regionwith a conventional stent in place;

FIG. 3 is a plot of force versus length for the conventional stent ofFIG. 2;

FIG. 4 is a fragmentary longitudinal cross-sectional view of a stenosisin a narrowing vessel region;

FIG. 5 is a plot of force versus length of an improved stent forplacement in FIG. 1;

FIG. 6 is a plot of force versus length of an improved stent forplacement in FIG. 4;

FIG. 7 is a side view of a self expanding stent having more wires perunit length at longitudinal center;

FIG. 8 is a side view of a self-expanding stent coil more closely spacedtoward center;

FIG. 9 is a side view of a self-expanding stent having thicker elementstoward longitudinal center;

FIG. 10 is an end view of the stent of FIG. 9;

FIG. 11 is a wafer view of the stent of FIG. 9;

FIG. 12 is a longitudinal profile of an alternate embodiment of theinvention in which the diameter is nonuniform along the stent length;

FIG. 13 is an enlarged view of element junctions in a self-expandingstent;

FIG. 14 is an enlarged view of an element junction in the self-expandingstent of FIG. 13;

FIG. 15 is an enlarged view of an element junction of a self-expandingstent;

FIG. 16 is a side view of a self-expanding stent having a greaterdensity of elements toward one end; and

FIG. 17 is a side view of a self-expanding stent having more closelyspaced elements toward one end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a stenosis 30, forming narrowed region 34, in avessel 31 within vessel wall 32. Adjacent to stenosis 30 is a healthyvessel region 36. FIG. 2 illustrates a conventional stent 40 in placeacross stenosis 30, out of the blood flow channel as indicated at 44.Stent 40 includes a stent end 44, shown angling into healthy vessel area36 at 38. Stent 40 as shown, has sufficient force to keep vessel 30 openagainst the rebound force of stenosis 30, and has more force thanrequired at stent end 42, resulting in stent 40 angling into the healthyvessel wall at 38. FIG. 3 illustrates an idealized plot 50 of outwardradial force, F, against stent length, L, for a conventional stent suchas that illustrated in FIG. 2. As shown, the force is substantiallyconstant over the length.

FIG. 4 illustrates a narrowing vessel 52 having a wide region 56, anarrowed region 58, and a stenosis 54. The narrowing vessel of FIG. 4illustrates the geometry as found in an ostium such as the left coronaryostium, where blood from the aorta flows into the left coronary artery.A stent with sufficient force to hold open wide region 56 would havegreater force than necessary to hold open narrowed region 58. A stenthaving the outward radial force axial distribution of FIG. 3, would haveinsufficient force at wide region 56 and greater than required force atnarrowed region 58.

FIG. 5 illustrates a plot 60 of outward radial force F along stentlength L for one stent embodying the present invention. The stent hasgreater force in a middle region 62 than at end regions 64 and 65. Astent having the force curve of FIG. 5 is suitable for bridging astenosis as illustrated in FIG. 1, while preventing the stent fromangling into healthy tissue as show in FIG. 2 at 38. FIG. 6 illustratesa plot 66 of outward radial force F along stent length L for anotherstent embodying the present invention. The stent has a greater force inend region 68 than at the opposite end region 70. A stent having theforce curve of FIG. 6 is suitable for bridging the stenosis asillustrated in FIG. 4, having sufficient force to hold open vessel wideregion 56 and less force in vessel narrow region 58, where less isrequired.

FIG. 7 illustrates a preferred embodiment of the invention producing aforce distribution as illustrated in FIG. 5. Self-expanding stent 80includes numerous resilient wires 82, interwoven as indicated at 88. Inuse, stent 80 is drawn longitudinally which increases the length anddecreases the diameter. Stent 80 is inserted into the distal end of thedelivery catheter, advanced to a stenosis to be crossed, and forced outof the delivery catheter distal end. Upon exiting the tube, stent 80expands radially and shortens axially, pushing against the stenosis andvessel walls.

Stent 80 includes a middle region 84 and end regions 86 and 87. Stent 80wires 82 are biased to resume the unconstrained state, which is widerand shorter than the constrained stent shape in the tube. The amount ofoutward radial force exerted per unit length of stent is greater inregions having a greater density of wires per unit length. Asillustrated in FIG. 7, stent 80 has a greater number of wires per unitlength in center region 84 than in end regions 86 and 87. Thus, stent 80has a greater outward radial force in center region 84 than in endregions 86 and 87. The greater number of wires per unit length in oneembodiment is the result of forming wires, which run the entire stentlength, more closely together toward stent center. In anotherembodiment, the greater number of wires is the result of adding morewires which only run in the center region of the stent.

FIG. 8 illustrates another embodiment of the invention in self-expandingstent 90, having a middle region 94 and end regions 96 and 97. Stent 90is formed of a single, spirally wound wire 92, forming a helix 98. Apreferred embodiment utilizes Nitinol material for wire 92. Helix 98 hasa distance between helix turns as indicated at 99. Distance 99 varieswith longitudinal position, being greater in middle region 94 and lessin end regions 96 and 97. Wire 92 is formed as a spring, biased toresume its unconstrained shape when released, after having beenstretched axially. The amount of outward radial force exerted is greaterin regions having more wire elements per unit length, which, in stent90, is achieved by having less space 99 between helix turns. Thus, stent90 has a greater outward radial force in center region 94 than in endregions 96 and 97.

FIG. 9 illustrates still another embodiment of the invention in stent100, having a middle region 104 and end regions 106 and 107. Stent 100has a tubular shape formed of a wire 102, which is shaped into severalsprings 108 having a zig-zag pattern, each spring 108 radiallyencircling a segment of stent 100, as indicated in FIG. 10. Referringagain to FIG. 9, springs 108 are longitudinally interconnected withsegments 109. Springs 108 and segments 109 in one embodiment are formedusing standard wire bending jigs and techniques, including brazingsegments 109 to springs 108. A preferred material for constructing stent100 is Nitinol. In another embodiment, springs and segments are formedby laser cutting a continuous-walled metallic tube, leaving only springs108 and segments 109.

FIG. 11 illustrates a wafer section in elevation taken along 11—11 inFIG. 10. Wire elements 102 are illustrated in cross section in middleregion 104 and end region 107. The element thickness in width and/orlength in end region 107, indicated at 101, is less than the elementthickness in middle region 104, indicated at 103. Middle elements havingthickness 103 can provide greater outward radial force than end elementshaving relatively lesser thickness 101. The radial expansive force canalso be varied by varying the frequency and/or amplitude of the zig-zagpattern.

FIG. 12 illustrates, in highly diagrammatic form, a phantom line profileof another embodiment of the invention. A profile of stent 110 is shownin phantom, having a middle region 114 and end regions 116 and 117.Stent 110 is formed, at least in part, from a shape memory material. Inthe preferred embodiment, stent 110 is formed of Nitinol. Shape memorymaterials can be annealed into a first shape, heated, thereby settingthe material structure, cooled, and deformed into a second shape. Thefirst shape has an average outside diameter greater than the second. Thematerial returns to the first, remembered shape at a phase transitiontemperature specific to the material composition.

FIG. 12 illustrates the stent shape to be remembered upon reaching bodytemperature. Stent 110 has a middle outside diameter 113 and end outsidediameter 111, where the middle outside diameter is greater than the endoutside diameter. Stent 110 can be compressed to fit within the deliverycatheter, the delivery catheter advanced to a stenosis, and the stentpushed out the delivery catheter distal end. Stent 110 then beginsresuming the remember shape of FIG. 12. The stenosed region typicallyhas the arcuate shape of FIG. 1. As stent middle outside diameter 113 isgreater than end outside diameter 111, and the vessel middle insidediameter is typically less than the vessel end inside diameters, stent110 can provide greater force in applying middle stent region 114against middle vessel walls than in applying end stent regions 117 and116 against the end vessel walls.

FIG. 13 illustrates another embodiment of the invention. In particular,FIG. 13 illustrates a tubular stent structure formed of elements meetingat junctions, where the junction size can be varied over the length ofthe stent. Stent 120 is shown having a structure 122 including elements124. Elements 124 intersect each other at junction 130 as illustrated indetail in FIG. 14. FIG. 15 illustrates a junction having a greateramount of material than the junction in FIG. 14. In the embodiment ofFIG. 15, junction 132 has a greater surface area than junction 130.Junctions having more material have greater capacity to provide radialoutward force than junctions having less material. One embodiment of theinvention has elements meeting or intersecting at junctions, where thejunctions have more material in the tube middle region and less materialin the tube end regions. In a preferred embodiment, the junctions areformed by laser cutting a Nitinol tube material.

In use, the tube can be compressed to fit within the delivery catheter,advanced to the stenosis, and pushed distally from the delivery catheterdistal end. As the tube regains its uncompressed shape, areas having agreater amount of material at the junctions are able to exert greateroutward radial force.

FIG. 16 illustrates an embodiment of the invention suitable for useacross stenoses in narrowing vessel regions, such as the left coronaryostium. Stent 140 has a first end region 147 and a second, opposite endregion 146. Stent 140 is similar to stent 80 in FIG. 7. The stent tubeincludes wires 142 which are wound around the stent and can beinterwoven. As illustrated in FIG. 16, wires 142 have a greater densityper stent unit length at second end region 146 than in first end region147. This enables second end region 146 to provide greater outwardradial force than first end region 147. Thus, first end region 147 canbe suitably matched for narrow vessel region 58, with second end region146 matched for wide vessel region 56.

FIG. 17 illustrates another embodiment of the invention suitable for useacross a stenosed, narrowing vessel region. Stent 150 extends from afirst end region 157 to a second end region 156. Stent 150 is similar inconstruction to stent 90 in FIG. 8, including wires 152 formed into ahelix or spiral 158. Helix turns are spaced a distance 159 apart. Asillustrated in FIG. 17, helix turns are spaced further apart at firstend region 157 than at second end region 156. This spacing allows stent150 to provide greater outward radial force at second end region 156than at first end region 157.

FIGS. 16 and 17 illustrate two embodiments having greater radial forceat one end than the other. This property can be produced using otherstructures. Another embodiment having this property is similar to alongitudinal half of FIG. 9, having a greater element thickness at oneend than the other. Yet another embodiment is similar to a longitudinalhalf of FIG. 12, having a greater outside diameter at one end than theother.

Stents providing greater outward radial force at one end than another,as in the embodiments of FIGS. 16 and 17, allow a stent to be placedacross a stenosis in a narrowing vessel region as illustrated in FIG. 4.The stent end having a greater radial force can expand into the widervessel region, while the stent end having lesser radial force can expandto the narrower vessel region wall, but with less force than if requiredto expand as far as the stent end in the wider vessel region. This canlessen unneeded force on the vessel wall while still holding the vesselopen and keeping the stent substantially out of the vessel flow path.

The present invention provides a stent having a radial force variedalong stent length. The stent has been described, in use, as bridgingstenosed vessel regions for illustrative purposes. Another use ismaintaining open channels through inflamed or otherwise restricted bodyconduits. Stents used for other purposes are explicitly within the scopeof the invention.

It should be noted that although self-expanding stents have been shownherein to illustrate the present invention, so called balloon expandablestents can also include the variable expansion force feature asdescribed herein. In the case of balloon expandable stents, however,these forces in general will be less than are necessary to expand thestent and thus the balloon will be used as known to those skilled in theart to complete the expansion of the stent. These balloon expandablestents may be advantageously deployed in bending areas of a vessels suchas at an ostium where a stent having thus rigid or heavy members isdesirable to enhance the flexibility of the stent. It should beunderstood therefore, that balloon expandable stents are also within thescope of the present invention.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many respects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size, and arrangement of parts without exceeding the scope of theinvention. The inventions's scope is, of course, defined in the languagein which the appended claims are expressed.

What is claimed is:
 1. A stent having a length, the stent expandable from an unexpanded state to an expanded state, the radius of the stent in the unexpanded state constant along the length of the stent, the stent comprising: a tubular shaped structure, said structure having a radially outward biased force, said force being varied along said length, said tubular structure having a first end region, a middle region, and a second end region, wherein said force is stronger in said middle region than in at least one of the first end region and the second end region, the structure including a plurality of interconnected non-abutting adjacent zig-zag bands, adjacent zig-zag bands connected by interconnecting elements.
 2. The stent of claim 1 formed of a material wherein the amount of material increases from the first end region to the middle region and from the second end region to the middle region.
 3. The stent of claim 1 formed of Nitinol.
 4. The stent of claim 1 wherein said force is stronger in the middle region than in the first end region and the second end region.
 5. The stent of claim 4 wherein the thickness of the zig-zag bands in the middle region is greater than the thickness of the zig-zag bands in the first end region and in the second end region.
 6. The stent of claim 1 wherein the width of the zig-zag bands in the middle region is greater than the width of the zig-zag bands in the first end region and the second end region.
 7. The stent of claim 1, each zig-zag band characterized by a frequency, the frequency of the zig-zag bands increasing from the first end region to the middle region and from the second end region to the middle region.
 8. The stent of claim 1, each zig-zag band characterized by an amplitude, the amplitude of the zig-zag bands increasing from the first end region to the middle region and from the second region to the middle region. 